1. Field of the Invention
The present invention relates to a silver halide emulsion and to a silver halide photographic light-sensitive material using the emulsion, which has high sensitivity, high graininess, and improved storage stability.
2. Description of the Related Art
A recent technical movement of a silver halide color photographic light-sensitive material aims at providing a light-sensitive material which can be used as (1) a light-sensitive material having very high sensitivity such as a photographing light-sensitive material of ISO 1600 corresponding to needs of attaching importance to sensitivity or (2) a light-sensitive material having satisfactory graininess, sharpness, and color reproducibility even for use in photographing performed by a small-formatted camera such as a 110-size system or disc-size system.
A technique of increasing sensitivity of a silver halide emulsion has a great advantage in improvements in graininess since a grain size for obtaining a certain level of sensitivity can be decreased.
In order to increase the sensitivity of a silver halide emulsion it is required that, (1) the number of photons absorbed by a single grain is increased, (2) an efficiency of converting photoelectrons generated by light absorption into a silver cluster (latent image) is increased, and (3) development activity for effectively utilizing the obtained latent image is increased. Increasing the size increases the number of photons absorbed by a single grain but degrades image quality. Increasing the development activity is an effective means of increasing the sensitivity. In the case of parallel development such as color development, however, the graininess is generally degraded. In order to increase the sensitivity without degrading graininess, it is most preferable to increase the efficiency of converting photoelectrons into a latent image, i.e., increase a quantum efficiency. In order to increase the quantum efficiency, a low-efficiency process such as recombination and latent image dispersion must be minimized. It is known that a reduction sensitization method of forming a small silver nucleus without development activity inside or on the surface of a silver halide is effective to prevent recombination.
In addition, James et al. found that when a certain type of reduction sensitization in which a coated film of an emulsion subjected to gold-plus-sulfur sensitization is vacuum-deaerated and heat-treated in a hydrogen gas atmosphere is performed, sensitivity can be increased at a fog level lower than that in conventional reduction sensitization. This sensitizing method is well known as hydrogen sensitization and effective as a sensitivity increasing means in a laboratory scale. This hydrogen sensitization is actually used in the field of astrography.
The method of reduction sensitization has been studied for a long time. Carroll, Lowe et al., and Fallens et al. disclose that a tin compound, a polyamine compound, and a thiourea dioxide-based compound are effective as a reduction sensitizer in U.S. Pat. Nos. 2,487,850 and 2,512,925 and British Patent 789,823, respectively. Collier compares properties of silver nuclei formed by various reduction sensitization methods in "Photographic Science and Engineering", Vol. 23, P. 113 (1979). She adopted methods of dimethylamineborane, stannous chloride, hydrazine, high-pH ripening, and low-pAg ripening. Reduction sensitization methods are also disclosed in U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777, and 3,930,867. Not only selection of a reduction sensitizer but also improvements in a method of using the reduction-sensitizer are described in JP-B-57-33572 and JP-B-58-1410 ("JP-B-" means examined published Japanese patent application) and JP-A-57-179835 ("JP-A-" means unexamined published Japanese patent application).
Although these hydrogen sensitizing and reduction sensitizing techniques have a sensitizing effect, however, they also have the following drawbacks. That is, as pointed out in JP-A-57-115539, some or all of silver nuclei produced through a reducing environment are decomposed in an oxidizing environment. In addition, as pointed out in "Photographic Science and Engineering", Vol. 19, page 50, when an emulsion is left to stand in the air at room temperature after it is hydrogen-sensitized, its sensitivity is sometimes decreased while its fog is increased.
In addition, the fact that a sensitizing effect obtained by reduction sensitization is lost by a heat treatment is also reported. Proposals of eliminating these drawbacks are disclosed in JP-A-57-115539 and JP-A-60-178445, but these means have not reached a sufficient level. Furthermore, its improvement in storage stability is directed to prevent degradation in photographic properties caused by heating. Therefore, an improvement in storage stability at around room temperature for a long time period is especially insufficient.